ICMP Messages

ICMP Messages

The Internet Control Message Protocol (ICMP) is a special-purpose message mechanism added to the TCP/IP suite that lets computers and routers in an internetwork report errors or provide information about unexpected circumstances. Remember that IP is a connectionless protocol and as such, contains no procedures that help to monitor successful packet delivery or test connectivity. Hosts use ICMP to send error messages to other hosts.

ICMP messages include the following types:

Message	Characteristics
Echo	The ICMP echo message is used to discover hosts and networks, and to verify that they are reachable. The ping utility is a popular utility that uses ICMP echo messages.
Destination unreachable	The destination unreachable message is sent if a packet cannot reach its destination for a variety of reasons. It might indicate the host is unavailable, or that there were problems detected in the packet header.
Time exceeded	The time exceeded message is sent when the packet's time-to-live (TTL) counter has expired.
Redirect	The redirect message is sent from a router to the sending device to indicate that a different route should be chosen for the packet. The redirect message can be sent if a better route is in the router's table, or if the selected route is unavailable or congested.
Source quench	The source quench message is sent by a receiving device to indicate that the flow of packets is too fast. When a sending device receives a source quench message, it slows its rate of transmission.
Router discovery	The router discovery message is a special broadcast message sent by hosts to discover the routers on a network. Routers respond to the message indicating their presence. They do not exchange routing information, but simply announce their availability.

 

 

 

Credit: Testout 640-802 CCNA Notes